1. Field of the Invention
The present invention relates to a perfecting press apparatus for forming images on both sides of a transfer medium.
2. Background Art
An electrophotographic system is one of most known printing systems used to copiers or printers. For example, as shown in FIG. 2, in a printing apparatus using a photosensitive drum 1 as an image carrier having a characteristic of charging negative polarity, firstly the photosensitive drum 1 is uniformly charged with negative polarity by a charger 2. Secondly, a light is irradiated by an exposure unit 3 to the photosensitive drum 1 on the basis of image information so as to form an electrostatic latent image, and a toner 5 is supplied to the electrostatic latent image by a developing unit 4 so as to form a toner image on the photosensitive drum 1. The toner image formed on the photosensitive drum 1 is transferred onto the transfer medium 7 such as a paper by a transfer device 6.
In a color image printing apparatus, such a structure has been proposed that, for example, as shown in FIG. 3, there are arranged four image forming units which are provided with the photosensitive drum 1, the charger 2, the exposure unit 3, and the transfer device 6, and the respective image forming units are mounted with developing units 41 to 44 supporting therein different color toners 51 to 54, and the toners of four colors are transferred to the transfer medium 7.
As a structure for printing color images on both sides of the transfer medium, for example, JP-A-7-77851 proposes as shown in FIG. 4, that the image forming units are alternately arranged at both sides holding the transfer medium 7 therebetween for carrying out color image printings.
In this case, immediately before the third image forming unit, a pair of chargers 101a, 101b are disposed for controlling charging polarity of the toner on the transfer medium 7, and the pair of chargers are disposed upstream of the subsequent respective image forming units. Accordingly, as shown in FIG. 4, in a tandem type printing apparatus for transferring the toners of four colors at one side for perfecting press, six pairs (101a, 101b to 601a and 601b) of chargers are required in total.
However, one of problems involved with the structure of FIG. 4 is that polarity of the toner deposited on the transfer medium 7 is inverted by transferring current of the image forming units for forming images on the opposite side. For example, when using the toner of negative charge, the toner transferred to the transfer medium 7 in the first image forming unit has negative polarity. When forming an image on the opposite side in the second image forming unit, however, the toner transferred to the transfer medium 7 in the first image forming unit turns out the positive charge, since the toner is given a positive charge by a transfer device of the second image forming unit. In case the toner inverted to the positive polarity is introduced into the third image forming unit, positive charges are repulsed in the transferring process of the third image forming unit. Since the toner deposited by the first image forming unit receives power moving toward the photosensitive drum of the third image forming unit, the charge of the toner on the transfer medium 7 must be returned to the negative polarity just before the third image forming unit.
As a method of not using a charge controlling instrument for inverting the charge of the toner on the transfer medium 7, for example, it is sufficient that polarity of the toner forming an image on an upper face and polarity for a rear face are made different. In this case, since the transferring current of the unit forming the image on the rear face has the same charging polarity as that of the toner for the upper face, the polarity of the toner is not inverted. However, two kinds of positive charges and negative charges per four colors must be developed.
There is disclosure in JP-A-7-77851 as to current applied to a charge controlling instrument. According to the Official Gazette, the charge controlling instrument is placed 3 to 10 mm apart from the transfer medium, and the current applied to the charge controlling instrument ranges 1 to 10 xcexcA/cm (preferably 2 to 5 xcexcA/cm) in response to properties of papers to be transferred. JP-A-7-77851 also describes as to current applied to the transfer unit, according to which the transfer device is placed 3 to 10 mm apart from the transfer medium, and the current applied to the charge controlling instrument ranges 1 to 10 xcexcA/cm (preferably 2 to 5 xcexcA/cm) in response to the properties of the papers to be transferred.
However, no description is made to the relation between current applied to the charge controlling instrument and current to the transfer unit. There can arise a problem in which retransfer phenomenon that the toner on the transfer medium returns to the photosensitive drum becomes remarkable.
Assuming that currents applied to the transfer device and the charge controlling instrument are Itt and Itc, and currents flowing owing to discharge therefrom in the direction toward the transfer medium are Ipt and Ipc, Ipt and Ipc contribute to transfer or charging of the toner on the transfer medium are.
Accordingly, it is an object of the invention to offer a perfecting press apparatus, enabling to suppress to a minimum the retransfer phenomenon that the toner on the transfer medium returns to the image carrier as the photosensitive drum.
To achieve the object, the invention provides a perfecting press apparatus, which includes: at least a first image forming unit, a second image forming unit and a third image forming unit; and a charge controlling unit of corona discharging type.
Each of the first, second and third image forming units includes: an image carrier; a charging unit for evenly charging the image carrier; an exposure unit for making image exposure on the charged image carrier on the basis of image information to thereby form an electrostatic latent image on the image carrier; a developing unit for supplying developer to the image carrier holding electrostatic latent image to thereby form a toner image on the image carrier; and a transfer unit of corona discharging type for transferring the toner image formed on the image carrier to a transfer medium.
The image carriers of the first image forming unit, the second image forming unit and the third image forming unit are alternately arranged at both sides of the transfer medium.
The charge controlling unit controls charging polarities of the toner images formed on both faces of the transfer medium by the first and second image forming units before the transfer medium is sent to the third image forming unit.
An absolute value of current flowing from the charge controlling unit in the direction of the transfer unit is made larger than an absolute value of current flowing from the transfer unit of the third image forming unit in the direction of the transfer medium.